AD7689 doesn't response

Hello,

   recently I've been working with SPI programming for AD7689. I am using 8-channel single ended input configuration, external reference (3.3V), full bandwidth, and disabled sequencer. The SPI timing mode is RAC(read/write after conversion), without a busy indicator.

   According to tests, all the power supply networks are on the correct voltage. The CS, SCLK, MOSI signals measured by the oscilloscope are as shown in the Figures. The MOSI is 0xF5C4 (0b1111,0101,1100,0100) in the Figure 5. The first 14 bits mean like this: overwrite; Unipolar, referenced to GND; channel 2; full bandwidth; external reference, temperature disabled; sequencer disabled; do not read back.

   The problem is: I never get any signal from SDO. SDO is low forever. Therefore, the master always reads 0 from AD7689.

    I don't know if there's any thing wrong with the SPI signal, or the reference voltage, or the pin configuration.

Fig 1. Power supply

Fig 2. Pins connection

Fig 3. CNV (yellow) and SCK (blue). This should be showing that the conversion time is enough.

Fig 4. CNV (yellow) and SCK (blue).

Fig 5. DIN (yellow) and SCK (blue). The master device sends 0xF5C4

   Thank you so much for any help!

   Walt YE